Turning down a request from a colleague or a boss can be difficult, especially if you’re worried about offending or upsetting the person. But saying no doesn’t have to seem like a personal affront. Have empathy for the situation your coworker is in — now they may have to make the time to do the task, or find someone else to take it on. The “no” will be much easier for your colleague to accept (and harder to object to) if you provide a clear but short explanation for why you can’t do what you’ve been asked to. You might already have too much on your plate, or feel that you can’t do the task well. Whatever the reason, be honest, polite, and straightforward.
Source: HBR